Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps

Abstract

Methodologies for conducting surface geochemical surveys and measuring the hydrocarbon flux rates of hydrocarbons migrating to the surface are addressed with examples from natural seeps and from anthropogenic seepage from underground gas storage reservoirs, leaky well casings, and underground coal gasification reactors. Natural gas flux was monitored for 1 year at Arrowhead Hot Springs, San Bernardino County, California, as part of an earthquake prediction program. The hot spring is on a splay of the San Andreas fault and releases 40 mL/min of free gases containing helium, hydrogen, light hydrocarbon gases, and radon. The volume of released gases varied by a factor of two within 7 months. Changes in gas flux could be a precursory signal of earthquake activity on the locked southern section of the fault and demonstrated that rapid changes were related to tectonic activity along this major basement fault. Gas flux associated with pressure changes in underground storage reservoirs confirms the rapid variations observed for natural seeps. Other changes in gas concentrations over a propane storage cavern are related to barometric and meteorologic variations. The rapidity with which natural gas can migrate through the earth was also demonstrated by measuring the gas flux in 122 boreholes over an underground coal gasification reactor. Baseline gas concentrations were established one month before the 180-m- (600-ft-) deep retort was pressured and fired. Leaked gases were detected at the surface in 2 to 15 days, depending on the location of the boreholes with respect to the retort at depth. The underground coal gasification (UCG) reactor provided an outstanding vehicle for migration flux measurements because of the unique gases generated in the reactor. Also, pressure and compositional changes in the reactor occur at known times in direct response to operational procedures. Individual gas pulses exhibited chromatographic effects as the gases migrated away from the source at depth. These chromatographic changes existed for only a few hours at the onset of a pressure pulse in the subsurface reactor and quickly returned to steady-state conditions in which the composition of the reactor gases matched those escaping at the surface.